1. Field of the Invention
The present invention relates to an organic contamination prevention method for maintaining the cleanliness of an electronic device substrate such as a cleaned semiconductor silicon wafer or a liquid crystal glass substrate, and also relates to an electronic device substrate that has undergone organic contamination prevention treatment using the method.
2. Description of the Related Art
In semiconductor manufacturing processes, it was recognized early that organic contamination of the wafer surface was deleterious. The importance of reducing organic contamination to a minimum has become even more pronounced with the development of LSI ultra high integration, and the road map announced by the U.S. Semiconductor Industry Association incorporates an item regarding the amount of organic carbon. In 1997 the reported value was 1×1014 carbon atoms/cm2 for a DRAM 256 Mbit, but this figure reduces to 6×1013 for a 1 Gbit in 2001, 3.5×1013 for a 16 Gbit in 2005, and 1.8×1013 for a 256 Gbit in 2009.
Large amounts of organic material exists in the atmosphere of semiconductor clean rooms, even in the case of ultra LSI, and the amount of this organic material exceeds 100 μg/m3 according to a variety of published reports. It is now known that when a silicon wafer is exposed to this type of atmosphere, organic material which is deleterious to the device manufacture is adsorbed onto the silicon surface, and analysis methods such as thermal desorption and gas chromatogram/mass spectrometry confirm this problem. Even in those cases where a substrate is stored inside a wafer case, outgas from the case material leads to the same type of organic contamination.
In order to remove organic contamination on a wafer surface, conventionally wet cleaning methods such as SC-1 treatment (composition of a standard cleaning solution is 1 part NH4OH: 1 part H2O2 :5 parts H2O by volume, and typical conditions involve immersion for 10 minutes at 70° C.), or pirhana treatment (composition of a standard cleaning solution is 4 parts H2SO4: 1 part H2O2 by volume, and typical conditions involve immersion for 10 minutes at 130° C.) have been deemed the most effective. These types of treatment are generally conducted in a dust free environment, in a cleaning apparatus installed inside a so-called clean draft, but as described above such environments also incorporate organic material, and so organic contamination re-occurs following cleaning.
According to a number of academic reports, the majority of organic contamination which occurs when a wafer is exposed to the atmosphere inside a semiconductor clean room is due to dioctyl phthalate (DOP) or dibutyl phthalate (DBP). It has been reported that contamination of a wafer with DOP equivalent to 1×1014 carbon atoms/cm2 in the cleaning apparatus used for treatment prior to gate oxidation, results in a large reduction in the yield of high precision devices.
Recently, many academic publications have reported the removal of organic contamination by cleaning at room temperature with pure water incorporating several dozen ppm of ozone, and a reduction in the surface carbon atom concentration to a value in the order of 1012 atoms/cm2 is reported as possible. However, the chemical oxide film generated by ozone displays quite strong chemical activity and the adsorption of organic matter thereto occurs extremely quickly, as disclosed, for example, in the 1996 spring proceedings of the Japan Society of Applied Physics, 27p-F-12. This phenomenon is explained on the basis of the strong polarity displayed by a hydrophilic silicon surface with a chemical oxide film, with organic material which incorporates polar groups, such as DOP, therefore adsorbing readily. Because similar chemical oxide films are also formed on the cleaned wafer surface in both SC-1 treatment and pirhana treatment, organic contamination from the atmosphere will also occur readily in these cases, via the same mechanism.
In those cases where a wafer is stored inside a wafer case, the only difference is that the organic materials which undergo adsorption are materials originating from additives in the case material such as butyl hydroxytoluene (BHT), which being a polar organic compound, will also adsorb readily onto the hydrophilic silicon surface.
Examples of cleaning methods in the academic literature which enable particularly large reductions in the organic carbon concentration on a wafer surface include dry treatments such as thermal oxidation, or treatment with ultraviolet radiation and ozone. However, even if the cleaning process is successful, in the same manner as a chemical oxidation film surface generated by the wet methods described above, the surface is still prone to organic contamination from the surrounding atmosphere. For example, according to the 1997 fall proceedings of the Japan Society of Applied Physics, 3p-R-2, adsorption of DOP from the atmosphere onto a thermal oxidation film surface exposed to the atmosphere of a clean room is several times the amount of contamination of an exposed bare silicon surface, while adsorption onto a surface treated with ultraviolet radiation and ozone is reported as being similarly high.
The installation of chemical filters, in which the main scavenger is activated carbon, in clean rooms, clean benches and clean drafts to enable the handling of wafers within a sufficiently clean environment is also being tested.
Electronic device substrates in a clean room manufacturing process are almost always stored inside a plastic case. After normal cleaning and immediately following rinsing and drying, when the surface activity is particularly high, the device is moved to the case. Conventionally these cases have been constructed of polypropylene for a number of reasons, including cost. As was described above, BHT is the major concern of the organic outgases produced from the polypropylene material. Consequently, a variety of improvements have been reported, and polypropylene cases with significantly reduced organic contamination are commercially available, although cases of polycarbonate, which is reported as a superior case material are not yet available.
As is evident from the road map mentioned above, organic carbon concentration on a wafer surface needs to be reduced to approximately 2×1013 atoms/cm2 by the year 2010. By using conventional mechanisms for removing deleterious organic impurities from the atmosphere and conducting the cleaning treatment in a dust free clean draft, and provided the clean room in which the wafer is exposed is of the same level of cleanliness, then it should be possible to reduce organic carbon on the wafer surface to the above level even with the aforementioned SC-1 or pirhana cleaning treatments. However, such a reduction would require installation of a plurality of chemical filters, such as activated carbon filters, for removing organic contamination, and moreover would require a thorough organic contamination removal system which was capable of supplying cleaned air at a sufficient flow rate.
However, the atmosphere in a semiconductor clean room incorporates many organic materials which will not adsorb to a wafer, but will be adsorbed onto activated carbon, and so the installed activated carbon chemical filters would deteriorate unexpectedly, and suffer from a short life span. Moreover, chemical filters are expensive, and so from an economic viewpoint, it would be desirable to develop a system which did not require the use of chemical filters.